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Currently we have a negative cpucap which describes the *absence* of
FP/SIMD rather than *presence* of FP/SIMD. This largely works, but is
somewhat awkward relative to other cpucaps that describe the presence of
a feature, and it would be nicer to have a cpucap which describes the
presence of FP/SIMD:
* This will allow the cpucap to be treated as a standard
ARM64_CPUCAP_SYSTEM_FEATURE, which can be detected with the standard
has_cpuid_feature() function and ARM64_CPUID_FIELDS() description.
* This ensures that the cpucap will only transition from not-present to
present, reducing the risk of unintentional and/or unsafe usage of
FP/SIMD before cpucaps are finalized.
* This will allow using arm64_cpu_capabilities::cpu_enable() to enable
the use of FP/SIMD later, with FP/SIMD being disabled at boot time
otherwise. This will ensure that any unintentional and/or unsafe usage
of FP/SIMD prior to this is trapped, and will ensure that FP/SIMD is
never unintentionally enabled for userspace in mismatched big.LITTLE
systems.
This patch replaces the negative ARM64_HAS_NO_FPSIMD cpucap with a
positive ARM64_HAS_FPSIMD cpucap, making changes as described above.
Note that as FP/SIMD will now be trapped when not supported system-wide,
do_fpsimd_acc() must handle these traps in the same way as for SVE and
SME. The commentary in fpsimd_restore_current_state() is updated to
describe the new scheme.
No users of system_supports_fpsimd() need to know that FP/SIMD is
available prior to alternatives being patched, so this is updated to
use alternative_has_cap_likely() to check for the ARM64_HAS_FPSIMD
cpucap, without generating code to test the system_cpucaps bitmap.
Signed-off-by: Mark Rutland <mark.rutland@arm.com>
Reviewed-by: Mark Brown <broonie@kernel.org>
Cc: Suzuki K Poulose <suzuki.poulose@arm.com>
Cc: Will Deacon <will@kernel.org>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>
34f66c4c4d